As more and more Americans continue to receive the COVID vaccine, there have been reports of some experiencing more side effects after having been administered the second dose.
While about a third of people who have received the vaccine report having flu-like symptoms, NBC News medical contributor Dr. Kavita Patel says that is a typical immune response.
"The second vaccine (dose) — think of it as having that hit to your immune system, and your immune system now recognizes the vaccine, so it does its job," Patel told Today.
The second dose is, in effect, working more like a booster dose.
"The immune system is seeing the vaccine for the first time with the first dose and is reacting to that, and the cells of the immune system are recruited to kind of recognize that spike protein (the part of the coronavirus that the vaccine affects),” added Dr. Bill Moss, a professor of infectious disease epidemiology at the Johns Hopkins Bloomberg School of Public Health.
“So when the body's immune system sees (the vaccine) a second time, there are more cells and there's a more intense immune response, resulting in those side effects."
But health experts don’t want people to assume they will definitely feel side effects.
"Just be prepared," Patel warned. "If you don't have a reaction, you don't need to worry that it didn't work. Every human and body is different."
Last week, the Centers for Disease Control and Prevention published new data indicating the most common side effects from receiving the Moderna and Pfizer COVID-19 vaccines were headaches (22.4%), fatigue (16.5%) and dizziness (16.5%).
Other symptoms included muscle pain, fever, chills, joint pain, nausea and swelling at the site of the injection.
Of the approximately 7,000 people that reported feeling any side effects, 640 were considered serious, including 133 deaths, CNBC reported.